Semiconductor devices are generally produced from patterned substrates. Multiple dies are formed on a substrate, where each of the dies comprises feature patterns (such as contact holes, metal lines, etc.) formed in designated layouts. Semiconductor devices with desired functions are eventually formed from these dies or formed by using these dies as a template. The substrate may be a wafer or a mask.
As used herein, the term “pattern” will be used to refer to one or a group of specific feature patterns formed in a certain layout. Within one die, for example, a pattern may be formed in repeating periods, wherein each of the repeating periods has an identical pattern i.e. a set of identical feature patterns in the same layout. For patterns on a certain die of interest, the repeating period may be greater than or equal to one. As used herein, patterns formed in a repeating period greater than one, for example equal to or greater than two within the die, will be referred to as a repetitive pattern. On the other hand, patterns which are formed in a repeating period equal to one within the die will be referred to as a non-repetitive pattern.
A repetitive pattern may be formed in either a regular or irregular/random distribution over the die. Here, a regularly distributed pattern indicates an orderly layout of that pattern over the die, while an irregularly or randomly distributed pattern indicates a relatively unpredictable layout of that pattern over the die. Identical patterns on dies, repetitive or non-repetitive, may be repeated on a substrate. This can be understood with reference to FIG. 1 and FIG. 2 which schematically illustrate a regularly distributed repetitive pattern and an irregularly distributed repetitive pattern, respectively. As shown in FIG. 1, a die 100 comprises thereon a repetitive pattern 101, wherein the repeated patterns 101 are orderly arrayed on die 100. As shown in FIG. 2, a die 200 comprises thereon a repetitive pattern 201, wherein the repetitive patterns 201 are randomly distributed over die 200.
In the manufacture of semiconductor devices, the patterned substrates are inspected for defects so that successful production of acceptable devices can be achieved. Inspection of a patterned substrate can be carried out through various technologies, among which one large category is the image-based inspection tools. In an image-based inspection, one or more images of the substrate/die are obtained using an imaging apparatus. The optical source of the imaging apparatus may be, for example, a charged particle beam (ion beam, electron beam), a laser beam, etc. Then, the images are analyzed to identify the presence of defects on the substrate/die. By means of the images, target patterns are compared in various ways and/or against various references, so that presence of abnormalities in the target patterns can be determined from the images. For example, in the case of electron beam inspection, grey level images of the substrate/die are obtained using an electron beam imaging apparatus. Grey level profiles of the imaged (target) patterns are compared to identify the presence of abnormal grey level(s) which may be recognized later as defects.
The image-based inspection tools generally can be operated using either of two approaches. In one approach, a set of one or more dies of interest is compared with one or two reference dies. The target and reference dies comprise identical patterns, repetitive or non-repetitive, and in the case of repetitive patterns, the patterns can be regularly or irregularly distributed at corresponding locations on the target and reference dies, respectively. Each target die is compared against the reference die(s), pattern by pattern, to determine the presence of defect(s) on the target die. The reference die(s) may be another imaged die(s), such as for example one which is arbitrarily determined to be defect-free. Alternatively, the reference die can be provided from a database. In common practice, this approach can be performed by comparing two dies, with one being the target die and the other being the reference die. Alternatively, it can be performed by comparing three or more dies in a one-by-one fashion, wherein one common die is simultaneously compared against the other dies and wherein the common die is the target die to be inspected with the other dies serving as the reference and arbitration dies. For convenience of description, this approach will be referred to as the Random Mode inspection.
In the other approach, an area on the die(s) is selected for inspection, wherein the selected area should include a repetitive pattern which is regularly distributed. Recall that a repetitive pattern, by definition used herein, is formed in a repeating period equal to or greater than two within the die. Patterns from individual repeating periods are compared against each other so as to determine the presence of defects in the selected area. In particular, feature patterns from a corresponding location in an individual repeating period are compared against each other. If multiple areas are to be inspected simultaneously, each of the selected areas again should comprise identical repetitive patterns, and corresponding patterns from some or all selected areas may be put in comparison (e.g., compared) together. It is noted that the selected areas may be on the same die or different dies. For convenience of description, this approach will be referred to as the Array Mode inspection.
Generally, the Random Mode method is applied to the inspection of repeating dies wherein the target pattern on each die must be identical in the form of a repetitive or non-repetitive pattern. In the case of a repetitive pattern, the pattern can be regularly or irregularly distributed over the die but must be at the same location on each die. Inspection of the die is performed by comparing it, pattern by pattern, with a reference. The reference can be other imaged die, or a database graphic representing identical patterns as the die. In other words, for the Random Mode inspection to work, presence of a reference is a must.
The Array Mode on the other hand can only be used for the inspection of regularly distributed repetitive patterns. Orderly distributed repeating periods must be present on the die. In other words, there should be at least two identical patterns in regular distribution over the die for the Array Mode to work. Inspection of the die is performed by comparing target patterns from individual periods considered to be repeating periods. In some cases, the periods considered as repeating periods may be located on the same die or different dies.
In summary, adequate repetition of pattern within a single die is essential for the Array Mode inspection to be performed, while presence of one or more reference die/database graphics representing identical patterns as the die is essential to the Random Mode inspection.
The Array Mode is often preferred over the Random Mode in the inspection of a die having repetitive patterns (repeating period>1), because in the Array Mode only one die (the die itself) needs to be imaged, which is time saving. However, for a die having non-repetitive patterns (repeating period=1) the Array Mode inspection is no longer practical because there is no repetition of patterns. The Random Mode must be used, and a reference must be available to carry out the inspection.
Therefore, as would be understood by those skilled in the art, single die defect detection/inspection alone of a piece of a unique die having non-repetitive patterns is not possible for the Array Mode method as there is no repetition of pattern and is only possible for the Random Mode method when a database graphic of the die is available as a reference. However, the later sometimes raises the confidentiality issue on the customer side.
Another difficult scenario is single die inspection of a repetitive pattern which is irregularly distributed over the die of interest. The Array Mode method is not practical in such a case because the patterns are not orderly repeated, and as mentioned for the Random Mode method to work a database graphic of the die must be available as a reference.
In other words, the Random Mode inspection performed with a database reference is currently the only solution for single die inspection of non-repetitive or irregularly distributed repetitive patterns. Without a database graphic of the die of interest, neither the conventional Random Mode or Array Mode inspection is applicable for such a task.
Accordingly, each of the conventional Random Mode and Array Mode inspections has its own limitation. Therefore, it is desired at least to have a more flexible method, for example one which is able to eliminate the need for the reference die or database graphic of the die in the Random Mode inspection method, especially for single die inspection.